"Are encouraged to…" is too imprecise. The ability to round-trip application-defined (ie, "unknown" from the perspective of the specification) relations is essential for reliable business process integration.
Specify explicit conformance criteria for round-tripping application-defined relations.
Line 25f Part 1, section 9.1.9
ed
Proposed Disposition of DIS 29500 Comment DE-0149 (Modified: 2007-12-27) There is already a robust method for the preservation of business process information. The Custom XML data storage part, as defined in Part 1, §15.2.4, allows for the reliable storage of any data which needs to accompany the document. However, the description of Unknown Relationships in Part 1, §9.1.9, page 17, line 25 will be updated, as follows, to make this clearer: (Note the text inside [[…]] will be added in response to addressing comment DE-0126.) Conforming consumers shall not fail to load a document containing unknown relationships. Conforming producers [[that are also consumers]] are encouraged to can, but are not required to, roundtrip and preserve unknown relationships and their target parts. [Guidance: This subclause defines the general behaviour for the consumption of unknown relationships. A conforming producer that wishes to store custom business data in an Office Open XML document should use instead the known relationship type for Custom XML data storage parts, as defined in Part 1, §15.2.4. end guidance]
